The present invention broadly relates to electronic light sensing devices. More specifically, the present invention relates to an optoelectronic detector for detecting radiation synchronized with pulsed illumination and its use thereof for suppressing background illumination and phase shift calculation.
Electronic light sensing involves conversion of light into an electronically readable form. Optoelectronic detector has a light sensitive component. Light sensing component creates electron-hole pairs when light impinges on its surface. These electrons are subsequently integrated and readout to understand the characteristics of the light.
The light sensing devices can detect spatial distribution of impinging radiation from an object as well as gather information related to the distance of an object. Parameters such as intensity and phase of the modulated light reflected from an object measure the distance of a light-reflecting object. Light sensing devices that can measure such parameters are known as Photo mixing devices or gated views.
Light sensitive component of these Photo mixing devices are integrated in Charge Coupled Devices (CCD) technologies or in standard Complementary Metal Oxide Semiconductor (CMOS) processes. Both these technologies use the concept of photoelectric effect wherein light or photons interact with photosensitive materials, such as crystallized silicon, to create electron-hole pairs in the material.
CCD is a light-sensitive integrated circuit comprising a single detector cell or a one-dimensional or two-dimensional array of photocells each of which is a capacitor. It stores and displays data relating to an image by converting each picture element (also referred to as pixel) in the image into an electrical charge. Further, CCD based devices can add charges collected over several integration cycles. This leads to a better signal over noise ratio.
High fabrication cost and limited system integration on chip limits the use of CCD based devices in favor of CMOS based light sensing devices.
An optoelectronic detector based on CMOS technology comprises a single detector or one-dimensional or two-dimensional arrays of CMOS-based photodiodes/photogates and readout amplifiers.
Most known standard CMOS process based optoelectronic detectors, convert collected charges to voltage and buffer the voltage for readout. These optoelectronic detectors can implement a synchronous electronic shutter pixel that allows integration of charges corresponding to impinging light during very short integration times. This electronic shutter typically is a sample and hold switch. This switch requires sampling of free charges in the form of voltage, after every integration time. It is possible to add these voltages over multiple integration times to measure the free charges collected over multiple integration times. However, charge to voltage conversion introduces noise during each integration time. This noise gets added to the measured voltage in each integration time. Consequently, large amount of noise gets added leading to low precision in measuring the properties of impinged light.
Photogate based detector architecture overcomes this drawback of CMOS technology based optoelectronic detectors. In Photogate based optoelectronic detectors, photo generated charges are collected in a depletion region underneath the photogate. This depletion region is built by means of applying a suitable potential gradient to the photogate. This method of charge collection is similar to that in a CCD type detector.
After integration over a pre-defined time, using photogate architecture, collected charges are transferred to a readout node by means of changing the potential of the photogate. Photogate type devices feature the disadvantage of low charge transport efficiency compared to CCD type detectors.
Charge transport efficiency is the ratio of the number of electrons read by a sensing arrangement, coupled to the optoelectronic detector, to the number of electrons created in the photosensitive area of the optoelectronic detector. The number of electrons sensed by the sensing arrangement is lower than the number actually produced in the detector because a significant number of electrons are lost in their transport from the detector to the sensing instrument. This leads to poor measurements of light features such as phase separation and intensity. Therefore, quality of images produced using the optoelectronic detectors is not very good and often has low contrast and poor brightness.
The overall performance of both CCD and CMOS based optoelectronic detectors suffer from background illumination measured along with the actual signal, at the charge sensing devices. The background illumination can saturate the readout channel if its intensity is very high, or can deteriorate the contrast between the charge sensing devices. As a result, the precision for the detection of the phase and the spatial distribution of the impinging radiation is low.
In the existing CCD and CMOS based optoelectronic detectors, background illumination is suppressed by compressing the energy emitted by a light source in a short pulse and integrating the light only for short pulse's duration. Compression of energy involves reducing the on time of the light source as compared to the off time of the light source. This is effective for light sources with a low mean power relative to the background illumination but which can deliver pulse intensities above background illumination for short pulse times such as LED or LASER light sources.
The above method of background suppression requires modification (in this case compression) of the emitted light before the light impinges on the optoelectronic detector. There is no component in the optoelectronic detector that can contribute to reduction of background illumination. Thus, the amount of background illumination that is suppressed is limited, leading to imprecision in the detection of the properties of impinged light. Imprecision occurs as the effect of background illumination is also recorded with the properties of light.
Several patents disclose the use of abovementioned technologies for light sensing, some of which are explained hereinafter.
U.S. Pat. No. 5,856,667 titled “Apparatus and method for detection and demodulation of an intensity modulated radiation field”, assigned to Leica A G, Heerbrugg, Switzerland discloses a CCD based image sensor with multiple image sensing elements. For sensing the light coming from an image, the system switches between the various image-sensing elements. Switching enables recording different parameters of the light at the different image sensing elements.
PCT patent application no. 98/10255 titled “Method and device for determining the phase and/or amplitude data of an electromagnetic wave”, discloses a CCD based device for the calculation of phase and amplitude of an electromagnetic wave. The device collects electrons with the help of staircase shaped depletion region.
Although the abovementioned patents disclose light sensing devices, they suffer from one or more of the disadvantages cited earlier, i.e., low transport efficiency, background illumination and inaccurate measurement of light parameters.
Keeping the above discussion in consideration, there is a need for an invention that features the possibility of adding charges created during several integration times. Further, there is a need for an invention that can correctly detect the phase of impinging modulated radiation with respect to a reference phase by reducing background illumination of the impinging light. Also, there is a need for an invention that has a high transport efficiency and therefore, provides an increase in precision to detect the phase shift of the pulsed impinging radiation over other devices produced using standard CMOS technology.